A canonical tag, often written as `rel=canonical`, is an HTML element used to prevent duplicate content issues in search engine optimization (SEO). When multiple URLs serve the same or very similar content, search engines might struggle to decide which version to index and rank. The canonical tag tells search engines which URL is the preferred, or canonical, version of a page. This helps consolidate link signals and ensures that search engines focus on the most important page. It’s crucial for maintaining good website health and improving search engine performance by avoiding content duplication problems.